The present invention relates generally to a system and method for sealing plastic. More specifically, the present invention relates to a system and method for the ultrasonic welding of plastic materials.
It is known to use plastic to construct containers. In this regard, it is known to create a container by sealing together two layers of plastic. One method for sealing together layers of plastic is through ultrasonic welding. Ultrasonic welding can be performed using an ultrasonic welding horn.
A welding horn is generally shaped to have a welding edge contoured to the shape of the desired weld. An ultrasonic welding horn further includes a source of vibration which is usually piezoelectric. Activating the source initiates vibration which excites the mass of an ultrasonic horn to vibrate at an ultrasonic frequency. Because of equilibrium, the vibrational energy transferred to the mass of the horn must, in turn, leave the horn in the form of heat, sound waves or otherwise by affecting movement of another object in contact with the horn. Intermolecular friction transmits horn vibrations to the interface of two parts, heating the surfaces and causing material to flow.
A vibrating welding edge may be brought into contact with sealable plastic layers, causing them to melt. As a result, the melted layers become welded together, forming a seal at the weld.
A common application of ultrasonic welding is for encapsulating an object within sealed layers, such as between layers of plastic. For encapsulating an object, a horn generally has a welding edge which has an indentation that is larger than the object to be encapsulated. The object is positioned between plastic layers. The welding edge is then pressed against the plastic layers, receiving the object into the indentation, while the welding edge forms a seal around the object by melting together the layers.
During such a welding process, a traditional ultrasonic welding horn exposes the entire object being welded to significant amounts of ultrasonic vibration, sound energy and heat. Many objects that are sealed between plastic layers are inanimate and unaffected by such exposure. However, more sensitive objects are not so resistant.
In some surgical implantation techniques, cells may be sealed between plastic layers. Such sealed cell containing devices can be placed internally within the body of a patient for a variety of applications including gene therapy. Ultrasonic welding has been found to be an effective and economical way to seal cells within plastic layers. However, ultrasonic vibrational and sound energy from a traditional welding horn can damage or kill living cells. This damage can render the cells useless for the intended application. Thus, it has been a problem to perform an ultrasonic weld on capsules for living tissue without harming the tissue with vibrational energy.
Likewise, in other applications the vibrational energy from the ultrasonic horn can preclude ultrasonic welding as a feasible technique for creating plastic containers. For example, such is the case in creating certain medical devices that include membranes. A need, therefore, exists for an improved system and method for ultrasonic welding.